1. Field of the Invention
The present invention relates to a tool for boring a hole in an element and, more particularly, to a staged tool for boring a hole in a heated element, which boring proceeds by axial penetration and which boring involves deformation of the material of the element being bored.
2. Discussion of Background and Relevant Information
A number of difficulties may be encountered when attempting to form a hole in a solid material. It is frequently desired to form such holes, especially in metallic elements. Such holes have been formed by tools that punch into and/or through the material; such holes have also been formed by forming a small pre-hole in the element and then by guiding a relatively large tool through the pre-hole. Difficulties arise because of constraints based on the resistance and rheology of the material being deformed, as well as because of friction and lubrication problems.
Staged tools have heretofore been developed to form holes in solid material. Examples of such tools are shown and described in Russian patent document No. A-880,545; Japanese patent document No. A-54109056; French patent document No. A-552,043; and French patent document No. E-25550. Each of the tools described in these various patents successively includes, beginning at the forward ends of the tools, a front end portion, which is cylindrical or conical; a neck; and a working portion having a right straight section of revolution which increases in the direction opposite to that of the advancement of the tool. These tools can be utilized with their working surfaces in direct contact with the wall of the hole pierced in the element or with a lubrication layer interposed between their working surfaces and the wall of the hole. If such a lubricant is utilized, it is generally positioned in the inlet of a pre-hole bored in the element, as is described, for example, in French patent document No. A 2 067 226, French patent document No. A 1 130 759, and British patent document No. A 1 365 510 . Such lubrication systems have a shortcoming, that shortcoming being that the lubricant mass, e.g., a flexible sheet of glass, a sheet of glass fabric, or a cone of powder of an agglomerated glass, may be pushed, to a large degree, in front of the tool where it cannot properly perform its lubricating function.